Clean renewable sources of energy are required to overcome the rising energy demand of the coming decades. Solar energy can be considered to be a carbon-neutral energy source of sufficient scale to meet future global energy demand. Variability in local insolation, however, requires cost-effective storage of solar energy for its large scale deployment as a primary energy source. In nature, photosynthesis captures sunlight and converts it into a wireless current which is stored. Efforts have been made to duplicate natural photosynthesis in energy conversion systems that capture and convert solar energy.
One of the most promising schemes for the large-scale storage of solar energy is the electrochemical conversion of water—an abundant and noncarbonaceous resource—into dihydrogen and dioxygen fuels. Electrolysis of water, that is, splitting water into oxygen and hydrogen gases, is one such energy conversion process that is not only important for the production of oxygen and/or hydrogen gases, but for energy storage. Energy is consumed in splitting water into hydrogen and oxygen gases and, when hydrogen and oxygen gases are recombined to form water, energy is released.
Electrocatalysts provide low energy activation pathways that permit electricity-producing reactions to occur at a practical rate. In the context of the electrolysis of water, electrocatalysts are required to negotiate the proton-coupled electron-transfer steps and thermodynamic demands associated with the oxidation of water (Equations 1 and 2).2H2OO2+4e−+4H+Eanodic=1.23−0.059 (pH) V vs NHE4e−+4H+2H2 Ecathodic=0.00−0.059 (pH) V vs NHE
Crystalline materials have been believed to be effective electrocatalysts as these materials provide the regularity of a crystal lattice that gives rise to a higher conductivity and less charge recombination at defects. U.S. patent application Ser. No. 10/343,272 describes a process involving spray pyrolysis (the use of toxic chemicals and high temperatures) in the preparation of a photocatalytic polycrystalline film of iron oxide.
Amorphous alloys have also been shown to be potentially effective electrocatalytic materials as these materials have shown higher activities and selectivities than their crystalline counterparts for many catalytic transformations. Reasons for the effectiveness of amorphous alloys have been attributed to a greater number of randomly oriented bonds in an amorphous solid relative to a crystalline solid enabling a higher density of coordinated unsaturated sites for the facile adsorption of reactants. As well, the discontinuous nature of amorphous materials can increase the number of edges and terminal oxygens (and thus an enhanced coverage of reactive species) as well as structural flexibility to enhance dioxygen evolution.
U.S. patent application Ser. No. 12/486,694 describes the electrolysis of Co2+ in phosphate, methylphosphonate and borate electrolytes to prepare an amorphous highly-active water oxidation catalyst as a thin-film on a current collector.
Despite advances in the development of electrocatalysts, significant market penetration by commercial electrolyzers remains hindered by the absence of a commercially competitive catalytic material that exhibits low overpotentials and high current densities over prolonged time periods. Therefore, a need remains for the development of improved materials and devices that operate with increased energy conversion efficiency.
Further, while dopants, nanostructuring, co-catalysts, atomic layer deposition, and/or plasmonic materials are known to enhance the photocatalytic activity of hematite (an α-ferrous oxide), these methods are cumbersome, expensive and/or lead to variable and undesirable characteristics in the electrocatalyst. Alternate means to optimize the electronic properties of electrocatalysts are desirable.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.